Degasification of molten steel



y 1962 w. ARMBRUSTER ET AL 3,042,510

' DEGASIFICATION OF MOLTEN STEEL Filed Nov. 19, 959

United States atent Ufihce 3,942,510 Patented July 23, 1962 3,042,510 DEGASIFICATION OF MOLTEN STEEL Werner Armbruster, Hanan (Main), and Hermann Maas,

Hattingen (Ruhr), Germany, assignors to W. C.

Heraeus G.m.b.H., Hanan (Main), Germany, and Ruhrstahl Airtiengeselischaft, Witten (Ruhr), Germany, both corporations of Germany Filed Nov. 19, 1959, Ser. No. 854,087 Claims priority, application Germany Nov. 25, 1958 6 Claims. (Cl. 75-49) The present invention relates to the degasification of molten steel.

Many methods for the degasification of steel are known, including vacuum degasification, which is one of the most important and in which the steel is passed in a thin layer or in a finely divided state through an evacuated chamber in which a large part of the gases, inter alia, hydrogen, contained in the steel, are given off.

In some methods the evacuated degasification chamber is situated above the molten metal which is drawn upwardly thereinto through the vacuum, and these processes may be either discontinuous or continuous. In a continuous method which has become known, the steel to be degasified passes through a vacuum chamber in which it is conveyed by means of a scavenging gas, so that the scavenging gas is therefore used for a mechanical purpose. In this connection, it has already been proposed to use reducing gases as a scavenging gas. However, the employment of such a scavenging gas is hardly a suitable means for reducing the hydrogen content of the steel to extremely small amounts, because the scavenging gas rises in the form of bubbles and when height increases and therefore pressure decreases, the bubbles become increasingly larger and can only expel part of the hydrogen. This is particularly true of killed steels. On the other hand, rimmed steels evolve a large amount of carbon monoxide which produces a more pronounced scavenging efiect. After a cetrain time, the formation of carbon monoxide decreases and hence the scavenging action also.

In this respect good results have been obtained, when gases are fed in at at least two positions at different levels in the path into the degasification chamber. In that case, gas bubbles are formed not only at the lower feed point but also at the higher ones, and these bubbles draw the hydrogen on", removing it from the steel. When several gas feed points are provided, it has proved advantageous to feed in at the lower points an inert scavenging gas and at the higher points oxygen, at least occasionally. This is justified whenever the reaction produced by the oxygen is too strong and must be lessened to some extent. It may also be advantageous to feed in less gas at the lower supply point, since, once the movement of the molten metal is well under way, it is no longer necessary to feed so much gas as initially. When starting the flow of the molten metal, it is in most cases advantageous to use greater gas quantities at the lower as well as at the higher feed points.

In vacuum degasification, a reduced pressure prevails and therefore the reaction between the carbon present and the oxygen supplied gets well under way. Owing to the presence of bubbles which are produced during the operation the hydrogen can diffuse into the small bubbles at the prevailing reduced pressure. This enables optimum scavenging and degasification to be obtained.

The supply of gas is regulated so as to obtain a desired speed of ascent of the molten metal and the gas bubbles. In this connection it is particularly desirable that the speed of ascent of the small bubbles be relatively small in the upper part of the path into the degasification chamber. This allows the hydrogen to difiuse into the small bubbles.

In order to promote this effect, it has been found advantageous to allow the formation of small bubbles and to give the upper part of a pipe or tube used for the ascending molten metal, a larger cross-section than the lower part. This ensures that the mixture of molten metal and gas bubbles remains for a relatively long period under reduced pressure and therefore, the hydrogen has suffi cient time to move out of the molten metal and into the gas bubbles.

Nozzles are used for introducing the bubbles, which nozzles may be advantageously cooled and let into watercooled flanges of pipe attachments.

The accompanying drawing shows byway of example a device in which the molten metal to be degasified flows into an evacuated degasification chamber through one path and returns through another path into the container for the molten metal.

In the drawing, a degasification chamber 1 is evacuated by means of a pump unit 2. The base of the degasification chamber is provided with two pipes 3 and -4 of which pipe 3 serves as an inflow pipe for the molten metal which returns to a container 5 by way of the pipe 4. The flow of molten metal is indicated by the arrows shown in the drawing. Delivery devices, shown as annular devices 6, 7 and 8 in the drawing, are provided for the delivery of gas and are located on the inflow pipe 3. They are connected to a supply of gas by means of pipes 9, 10 and 11.

The degasification apparatus 1-4 is immersed into the molten metal from above. The pump unit is then put into operation and the molten metal therefore rises into the pipes 3 and 4. A gas is then delivered through the pipes 9, 10 and 11, which oxygen on the one hand starts the movement of the molten metal in the direction of the arrow and on the other hand produces the above mentioned chemical processes. When the movement of the molten metal is well under way, it is often possible to stop the feed of oxygen to the lower device 6, because a feed through 7 and 8 is in most cases sufficient in order to maintain the flow. However in other cases, an inert scavenging gas may be introduced, if the flow of metal has diminished too much for the purpose in question.

The introduction of oxygen gas into the melt causes the generation of carbon monoxide and water vapor, especially in the upper part of the inflow pipe 3. The gas bubbles become greater by the reaction of oxygen with the carbon content and with the hydrogen content of the steel melt. Besides that, if oxygen is introduced not only at a lower point, but also at a higher point into the inflow pipe, an additional chemical reaction occurs which transforms into carbon dioxide at least part of the carbon monoxide already formed. The reaction of transformation of the carbon monoxide into carbon dioxide is exothermic and the heat produced causes an additional heating of the steel. These reactions naturally do not occur if only inert gases are used. In this case only the transport and the scavenging of non-desired gases take place and the complete reaction, as defined above, is performed only by the described invention.

What we claim is:

1. In a method for degasifying molten steel in which the molten metal gives off at least part of the gases which are contained in it into an evacuated chamber situated above the surface of the molten metal and in which the molten metal is conveyed into the evacuated chamber through a first tube by aid of an introduced gas and returns through a second tube into the melt, the step of introducing gases into the first tube at a plurality of superimposed delivery points positioned at a zone diverging in the direction of gas flow.

2. A method according to claim 1, in which the delivery of gas is regulated so as to obtain an ascending speed of molten metal and gas bubbles which gives to the gases contained in the metal a suflicient time period for a difiusion into the bubbles.

3. Apparatus for degasifying molten steel comprising a container for molten metal, a vacuum chamber located above" said container, inflow and outflow pipes between said chamber and said container, said inflow pipe having a cross section which is larger at the upper part thereof than at the lower part, whereby the mixture of molten metal and gas remains a relatively long time under a reduced pressure, and means for introducing gases into said inflow pipe.

4. Apparatus as claimed in claim 3 in which cooled nozzles are used for introducing the gases into the inflow pipe.

5. Apparatus as claimed in claim 4, in which said nozzles are let into water-cooled flanges of the pipe attachments.

I at a higher level and in which it thereafter returns into said container through another outletpipe, the step of introducing gases into the inflow pipeat a plurality of superimposed delivery points positioned at a zone diverging in the direction of gas flow.

References Cited in the file of this patent UNITED STATES PATENTS 1,921,060 Wililams Aug. 8,-1933 2,093,666 Vogt Sept. 21, 1937 2,855,293 Savard et al. Oct. 7, 1958 2,893,860 Lorenz July 7,1959 

1. IN A METHOD FOR DEGASIFYING MOLTEN STEEL IN WHICH THE MOLTEN METAL GIVES OF AT LEAST PART OF THE GASES WHICH ARE CONTAINED IN IT INTO AN EVACULATED CHAMBER SITUATEED ABOVE THE SURFACE OF THE MOLTEN METAL AND IN WHICH THE MOLTEN METAL IS CONVEYED INTO TTHE EVACUATED CHAMBER THROUGH A FIRST TUBE BY AID OF AN INTRODUCED GAS AND RETURNS THROUHG A SECOND TUBE INTO THE MELT, THE STEP OF INTRODUCING GASES INTO THE FIRST TUBE AT A PLURALITY OF SUPERIMPOSED DELIVEERY POINTS POSITIONEED AT A ZONE DIVERRGING IN THE DIRECTION OF GAS FLOW. 